The present invention relates generally to electronic materials for microwave applications, and more particularly to such materials including ferroelectric materials having a tunable dielectric constant.
Tunable ferroelectric materials are the materials whose permittivity (more commonly called dielectric constant) can be varied by varying the strength of an electric field to which the materials are subjected or immersed. Even though these materials work in their paraelectric phase above Curie temperature, they are conveniently called “ferroelectric” because they exhibit spontaneous polarization at temperatures below the Curie temperature. Typical tunable ferroelectric materials are barium-strontium titanate (BST) or BST composites. Examples of such materials can be found in U.S. Pat. Nos. 5,312,790, 5,427,988, 5,486,491 and 5,643,429. These materials, especially BSTO-MgO composites, show low dielectric loss and high tunability. Tunability is defined as the fractional change in the dielectric constant with applied voltage. These unique properties make these materials suitable for microwave applications such as phase shifter, tunable filters, tunable resonators, and delay lines.
U.S. Pat. No. 5,830,591 discloses a multi-layer ferroelectric composite waveguide in which the effective dielectric constant of the waveguide can be reduced while maintaining tunability. The waveguide is constructed of high and low dielectric constant layers. The multi-layer waveguide is comprised of bias plates that are perpendicular to the laminate direction to maintain tunability in the structure. The structure disclosed in U.S. Pat. No. 5,830,591 is only suitable for waveguide applications. Since high dielectric fields, for example about 10 V/μm, are necessary to tune tunable material, especially in waveguide applications, the distance between bias electrodes should be kept small. With the bias plate arrangement of U.S. Pat. No. 5,830,591, multiple layers would be needed along the direction of that bias plates as well as in the direction of the laminated dielectric material stack. This makes fabrication of such devices complex.
U.S. Pat. No. 5,729,239 discloses a device for scanning in a scanning plane that includes a periodic array of conductive plates disposed along the scanning axis, adjacent plates being disposed about half a wavelength apart. The device has a periodic array of slabs disposed along the scanning axis, each slab comprising ferroelectric material, being disposed between a pair of adjacent conductive plates of the periodic array of conductive plates, with adjacent slabs being separated by one of the conductive plates. Each of the slabs has a receiving face and a radiating face substantially parallel to each other. Each of the slabs transmits an electromagnetic signal from the receiving face to the radiating face. Input transmission means feed an input electromagnetic signal to the periodic array of slabs in a propagation direction so that the input electromagnetic signal is incident on the receiving faces of each of the slabs and so that the electrical component of the input electromagnetic signal received at each receiving face has a component parallel to the scanning axis. Output transmission means transmit an output signal from the periodic array of slabs responsive to the electromagnetic signal transmitted from each receiving face in the corresponding slab. The device also includes a plurality of means for selectively applying a voltage across each of the pairs of conductive plates disposed about a slab so as to selectively control the phase of the electromagnetic signal received at each of the radiating faces having been transmitted from the receiving face in the corresponding slab.
U.S. Pat. No. 5,729,239 discloses the use of barium strontium titanate (BSTO), or composites thereof, or other ceramics as the ferroelectric material. BSTO-MgO has also been proposed by others for use as a tunable ferroelectric material. However, the materials in the BSTO-MgO system generally have dielectric constants of over 100. The high dielectric constant is not suitable for some microwave applications such as patch antennas, which lower the antennas' efficiencies. High dielectric constant materials also cause low characteristic impedance (<10 Ω) in microstrip, coplanar, and other planar structure transmission lines, which strongly limits the application of high dielectric constant materials. Low dielectric constant materials (for example, with dielectric constants less than 40) with low loss and high tunability are desired for patch antennas and other microwave applications.
It would be desirable to construct a ferroelectric structure having a relatively low overall dielectric constant that takes advantage of the high tunability and low loss characteristics of materials such as BST composites, having high dielectric constants. It is further desired to construct such structures for use in various microwave devices such as microstrips, coplanar or other planar microwave transmission lines, coaxial cable, or waveguides.